In order to suppress harmonics of an input signal, usually a PFC (Power Factor Correction) circuit is used in a switching power supply which performs AC (Alternating Current)-DC (Direct Current) conversion.
A PFC circuit generates current which is in-phase with AC input voltage and which is substantially sinusoidal. By doing so, the PFC circuit suppresses harmonics and improves a power factor.
Formerly the following technique was proposed. If a detected value of current outputted from a DC-DC converter which converts voltage outputted from a PFC circuit to determined voltage is smaller than a set value, then the operation of the PFC circuit is stopped.
Japanese Laid-open Patent Publication No. 09-201052
If there is a load fluctuation, such as a sudden increase in load current, during the operation of a switching power supply, voltage outputted from a PFC circuit may fall and become smaller than a determined value. This causes a decrease in the efficiency of the switching power supply.